1. Field of the Invention
The present invention relates to an ink jet recording apparatus for performing recording by ejecting ink from recording means onto a recorded medium.
2. Description of the Related Art
There are recording apparatuses having recording capabilities, such as printers, copiers and facsimiles, or recording apparatuses for use as output equipments of composite-type electronic equipment and work station including computers and word processors. The above-mentioned recording apparatuses are to record images (including texts and the like) on recorded media, such as sheets of paper or plastic thin sheets, on the basis of image information (containing character information). The recording apparatuses may be classified by recording method into ink-jet type, wire dot-matrix type, thermal, recording type and laser-beam type. Of the types of recording apparatuses, the ink-jet type recording apparatuses (ink jet recording apparatuses) perform recording by ejecting ink from recording means (recording head(s)) onto recorded media, which are superior to the other types of recording apparatuses and enable high-resolution recording to be carried out quietly at high speed. In addition, the ink-jet type recording apparatuses are inexpensive, thus displaying excellent characteristics.
In recent years, with the growing need for apparatuses capable of printing finer (high-resolution) color images, many color ink-jet recording apparatuses capable of recording color images by ejecting a plurality of color inks have also been developed.
To improve recording speed, the above-mentioned ink jet recording apparatuses use a recording head constituted of a plurality of recording elements integrated therein, with a plurality of combinations of ink ejection orifices as ink ejection parts and liquid channels integrated therein. For color recording, plural recording heads configured such above are generally provided.
FIG. 1 shows a configuration of a printer part for carrying out printing on the surface of a recording medium using the above-mentioned recording head. Indicated with the reference numeral 101 in FIG. 1 are ink cartridges, each of which is composed of an ink tank and a recording head 102. Each ink tank is filled with one of four color inks, namely black, cyan, magenta or yellow ink. A row of plural orifices is formed on the recording head 102. FIG. 2 is a schematic diagram showing the plural ejection orifices aligned on the recording head 102 as viewed from the direction of an arrow Z in FIG. 1. Indicated with the reference numeral 201 in FIG. 2 are orifices aligned on the recording head 102.
In FIG. 1, the reference numeral 103 designates a convey roller. The convey roller 103 rotates in the direction of the arrow while holding a recording paper P in cooperation with an auxiliary roller 104. The rotation of the paper feed roller 103 in cooperation with that of the auxiliary roller 104 enables the recording paper P to be conveyed along the Y direction as a sub-scan direction from the upstream side to the downstream side. The reference numeral 105 designates feed rollers, which are operative not only to feed the recording paper P, but also to hold the same therebetween in the same manner as between the rollers 103 and 104. The reference numeral 106 designates a carriage for supporting and moving the four ink cartridges during printing. The carriage 106 is placed at a home position (h) indicated with a dotted line during standby or when the recording head needs recovering.
The carriage 106 located at the home position before printing is initiated to move along the direction indicated with an arrow X in FIG. 1 in response to input of print start instructions. As the carriage 106 moves, recording is performed by ejecting ink from the plural ejection orifices 201 formed on the ejection orifice surface of the recording head 102 on the basis of recording data. After completion of printing data up to the rear end of the line, the carriage 106 returns to the home position and starts moving again in the X direction to print data for the next line.
In the case of image printing, various factors such as color development, gradation and uniformity are required. Uniformity is critical to image printing, especially when slight variations in nozzles occur in the process of manufacturing recording heads. It is known that the slight variations in nozzles affect the ink ejection amount and direction of each nozzle to cause density irregularities and the like, resulting in a decline in the quality of a finally printed image.
Referring to FIGS. 3 and 4, a specific example of such a decline in the quality of an image will be described. Schematically shown at (a) of FIG. 3 with the reference numeral 31 is a recording head in which a row of eight nozzles 32 is provided. The reference numeral 33 designates ink droplets (hereinbelow referred to as ink droplets or simply as the ink) ejected from the nozzles 32. Shown at (b) of FIG. 3 is an example of an image formed by ejecting the ink periodically from the row of the nozzles of the recording head 31. As shown, eight dots are recorded by ejecting the ink from each of the nozzles while moving the recording head. Further, (c) of FIG. 3 is a graph showing optical density of the image shown at (b) of FIG. 3, choosing density as the abscissa.
It is ideal that the same amount of ink droplets is ejected from the recording head 31 in the same direction as shown at (a) of FIG. 3. If such ideal ejection is attained, the same sized dots will be hit on the paper surface as shown at (b) of FIG. 3, thus obtaining a uniform image without any density irregularity as a whole as shown at (c) of FIG. 3.
However, if variations in nozzles occur as discussed above and printing is performed as shown in FIG. 3 without any compensation for the variations, the size and direction of the ink droplets ejected from the nozzles will vary to cause density irregularities.
FIG. 4 is a schematic diagram for explaining the density irregularities caused when the variations in nozzles occur in the recording head. In FIG. 4, (a), (b) and (c) correspond to (a), (b) and (c) of FIG. 3, respectively. In other words, FIGS. 3 and 4 show a comparison between ideal recording and an actual state in which recording is performed with nozzles varying from one another.
Shown at (a) of FIG. 4 is a state in which variations occurs in the recording head and hence in the size and direction of ink droplets ejected from the nozzles of the recording head. As shown at (a) of FIG. 4, if the size and direction of ink droplets ejected from the respective nozzles vary, the ink droplets may be hit on the paper surface as shown at (b) of FIG. 4. This cause indicates the existence of blank parts that do not meet 100 percent of the area factors periodically, or excess dots overlapped one upon another to the contrary, or the occurrence of a white streak as shown in the center. The group of dots hit in this state shows a density distribution as shown at (c) of FIG. 4 in the direction in which the nozzles are aligned, resulting in such a phenomenon that it is perceptible to the human eye as density irregularities. Variations in paper feed amount may also make the streak conspicuous.
To take measures against the above-mentioned density irregularities, Japanese Patent Laid-open No. 06-143618 discloses a method of reducing the density irregularities. Referring to FIG. 5, the method will be described below in brief.
The recording operation as shown in FIG. 5 is completed by performing scanning of the recording head 31 three times for a recording area as shown at (b) of FIG. 3 or 4. Shown at (a) of FIG. 5 with the reference numbers 31a, 31b and 31c are relative positions of the recording head 31 at the fist, second and third scans, respectively. In this recording operation, the recording head 31 and the recording paper are moved relative to each other along a sub-scan direction (the arrangement or alignment direction of the nozzles of the recording head in this drawing) each time the recording head is scanned. In this case, the amount of relative movement corresponds to that of half of the eight nozzles provided in the recording head 31.
According to the method, the recording operation shown in FIG. 5 is performed by performing scanning of the recording head 31 three times for the print area shown at (b) of FIG. 3 or 4. However, half of the print area, that is, the area corresponding to four nozzles is completed at two scans of the recording head (hereinbelow, referred to as a two-pass scan operation of the recording head). In this case, the eight nozzles of the recording head are divided into two groups, namely the upper four nozzles and the lower four nozzles. Dots to be printed by respective nozzles at one main scan represent data obtained by thinning out prescribed image data by about half according to a predetermined image data pattern. Then dots are embedded in the remaining half of the image data at the second main scan to complete a printed output for the four-pixel area. Such a recording method is referred to as a multi-pass recording method below. It should be noted that since the recording method shown in FIG. 5 is to complete a printed output for a predetermined area at two scans, it is also referred to as a two-pass recording method.
Such a recording method can reduce by half the influence of a peculiarity of each nozzle on a printed image even if the recording head used is the same as that shown in FIG. 4. As a result, the printed image comes out well as shown at (b) of FIG. 5 with less black or white streaks that became noticeable in the image shown at (b) of FIG. 4. Consequently, density irregularities can be considerably reduced as shown at (c) of FIG. 5, compared to those in FIG. 4. Upon performing this recording operation, the image data is divided according to a predetermined image data pattern so that both image data obtained at the first and second main scans will be complementary to each other. As shown in FIGS. 6A, 6B and 6C, the image data pattern most commonly takes the form of a zigzag or checker flag pattern in which image data dot the matrix every other unit of pixels. Therefore, in a unit print area (in units of four pixels in this case), printed outputs are completed by the first main-scan at which a checker flag pattern is printed and the second main scan at which a reverse checker flag pattern (reverse pattern to that of the first main scan) is printed. FIGS. 6A, 6B and 6C describe how the thinning-out checker flag and reverse checker flag patterns are used to perform recording for a fixed area. In FIG. 6, the reference numerals 31a, 31b and 31c also indicate relative positions of the recording head 31 at the first, second and third scans, respectively.
As shown in FIG. 6A, the lower four nozzles are used to record the thinning-out checker flag pattern at the first main scan. Then, at the second main scan, an amount of paper feed corresponding to four pixels (one-half the head length) is performed to record the thinning-out reverse checker flag pattern (FIG. 6B). Further, at third main scan, the same amount of paper feed corresponding to four pixels (one-half the head length) is performed to record the thinning-out checker flag pattern again (FIG. 6C). Thus the thinning-out checker flag and reverse checker flag patterns are alternately recorded while feeding the paper by four pixels at each scan, thus completing recording for the recording area in units of four pixels at each main scan.
As discussed above, an image is completed using two different kinds of groups of nozzles in the same area, which makes it possible to obtain a high-quality image without any density irregularity.
On the other hand, Japanese Patent Laid-open No. 06-135014 discloses a technique for recording high-quality images in which bleeding of ink is prevented at the border part of black and color images. The following describes this method in brief. When a black image to be printed with a black ink is contiguous to a color image to be printed with a color ink, only predetermined ejection part of the ejection group for black ink is used so that scanning (in the X direction) to form the black image and scanning (in the X direction) to form the color image will not be performed continuously, thereby achieving high-quality images in which bleeding of ink is prevented at the border part of the black and color images.
Japanese Patent Laid-open No. 06-135014, however, fails to make reference to reciprocating recording for performing recording when the recording head is scanned both in its going or forward direction and its returning or backward direction.
In studying color shading (or lack of uniformity in color) in the reciprocating recording operation using the above-mentioned multi-pass recording method, the following phenomenon was confirmed. Suppose that ink is ejected from at least two different kinds of ejection orifice rows to complete image formation on respective image areas. In this case, some color shading was found in overlapped part of the first image area to be completed by the first ejection orifice row and the second image area to be completed by the second ejection orifice row. In other words, the color shading was developed between image areas in the overlapped part, the image areas differing from each other in the time it took to perform reciprocating recording. The first image area completed by the first ejection orifice row varies with time in penetrating and fixing states to the recording medium. Therefore, if the ink ejected from the second ejection orifice row is overlapped on or contiguous to the first image area, the above-mentioned phenomenon may occur depending on how long it takes to be overlapped on or contiguous to the first image area. In other words, variations in penetrating and fixing states to the recording medium between both inks overlapped or contiguous with each other can cause the color shading.
Japanese Patent Laid-open No. 06-135014 also fails to make reference to the above-mentioned multi-pass recording method. In studying bleeding between colors in the process of multi-pass recording as well, it was found that the multi-pass recording method showed characteristics different from the one-pass recording method. In addition to the difference in bleeding, recording using the multi-pass recording method differed from recording using the one-pass recording method in the phenomenon that the recorded portion on the recording medium was made bleached-looking due to crowded droplets of plural color inks hit on the recording medium. The differences in these phenomena between the multi-pass recording method and the one-pass recording method are made due to the fact that the multi-pass recording method has not only the effect of preventing density irregularities, but also the effect of reducing the number of dots to be recorded per unit time, which makes different from the one-pass recording method in penetrating and fixing state to the recording medium.
The present invention has been made in view of the above-mentioned problems, and it is an object thereof to provide an ink jet recording apparatus capable of recording high-quality images in which not only color shading caused in the reciprocating recording operation is prevented, but also bleeding between colors inherent in multi-pass recording is reduced.
In attaining the above-mentioned object and according to the present invention, there is provided an ink jet recording apparatus or method for recording images on a recording medium. The ink jet recording apparatus includes scanning means using recording means having a first ejection orifice row and a second ejection orifice row, each of which has a plurality of ejection orifices aligned thereon for ejecting each ink. The scanning means moves the recording means relative to the recording medium to perform reciprocating recording along a main scan direction different from the direction in which the ejection orifices are aligned. The ink jet recording apparatus also includes sub-scanning means for sub-scanning the recording medium along a sub-scan direction from the upstream side to the downstream side. In this configuration, recording performed by ejecting ink from the recording means while the main scanning means is scanning the recording means, and sub-scanning performed by the sub-scanning means are repeated to record images on the recording medium. The above-mentioned ink jet recording apparatus or method features that when the number of times of record scans in the main scan direction to make the first ejection orifice row complete image formation is set to m (where m is a positive integer), the number of times of record scans in the main scan direction to make the second ejection orifice row complete image formation is set to n (where n is a positive integer), and the amount of sub-scanning of the recording medium performed by the sub-scanning means every main scan is set to L (where L is a positive number), the position of the top of the ejection orifices in the second ejection orifice row, to be used from the upstream side of the sub-scan direction is spaced a length of (m+a)xc3x97L apart from the first ejection orifice row to the downstream side of the sub-scan direction, and the value of a is controlled to make (m+a) an even number.
In another aspect of the present invention, the ink jet recording apparatus or method features that when the number of times of record scans in the main scan direction to make the first ejection orifice row complete image formation is set to m (where m is a positive integer), the number of times of record scans in the main scan direction to make the second ejection orifice row complete image formation is set to n (where n is a positive integer), and the amount of conveyance of the recording medium performed in the sub-scan direction every main scan is set to L (where L is a positive number), the position of the top of the ejection orifices in the second ejection orifice row, to be used from the downstream side of the sub-scan direction is spaced a length of (m+a)xc3x97L apart from the first ejection orifice row in the sub-scan direction, and the value of a is controlled to make (n+a) an even number.
In still another aspect of the present invention, the ink jet recording apparatus or method features that when the number of times of record scans in the main scan direction to make the first ejection orifice row complete image formation is set to m (where m is a positive integer), and the number of times of record scans in the main scan direction to make the second ejection orifice row complete image formation is set to n (where n is a positive integer), at least two kinds of amounts of sub-scanning of the recording medium performed in the sub-scan direction every main scan are used to determine, relative to the first ejection orifice row, the position of the top of the ejection orifices in the second ejection orifice row to be used from the upstream side of the sub-scanning performed by the sub-scanning means, wherein the position is equivalent to either of at least two kinds of total amounts of conveyance of the recording medium continuously performed (m+a) times using the at least two kinds of amounts of sub-scanning, and the value of a is controlled to make (m+a) an even number.
In yet another aspect of the present invention, the ink jet apparatus or method features that when the number of times of record scans in the main scan direction to make the first ejection orifice row complete image formation is set to m (where m is a positive integer), and the number of times of record scans in the main scan direction to make the second ejection orifice row complete image formation is set to n (where n is a positive integer), at least two kinds of amounts of sub-scanning performed by the sub-scan means every main scan are used to determine, relative to the first ejection orifice row, the position of the top of the ejection orifices in the second ejection orifice row to be used from the downstream side of the sub-scanning, wherein the position is equivalent to at least two kinds of total amounts of sub-scanning performed continuously (m+a) times (where a is a positive integer) using the at least two kinds of amount of sub-scanning, and the value of a is controlled to make (n+a) an even number.